Rendering image returns Null data
I didn't find the reason for the strange behaviour. There must be something wrong in the scene settings, maybe someone can investigate further.
But when I made a new scene, copied all objects from the old scene to the new one, everything rendered fine. Do not try to copy the old scene by using Full Copy, in this case the new scene won't render either.
This is just a workaround to help you rendering the animation. Hopefully someone finds a better answer.